Paper of specialized performance characteristics may be created by applying a thin layer of coating material to one or both sides of the paper. The coating is typically a mixture of a fine plate-like mineral, typically clay or particulate calcium carbonate; coloring agents, typically titanium dioxide for a white sheet; and a binder which may be of the organic type or of a synthetic composition. In addition, rosin, gelatins, glues, starches or waxes may be applied to paper for sizing.
Coated paper is typically used in magazines, commercial catalogs and advertising inserts in newspapers and other applications requiring specialized paper qualities.
Various devices have been employed in the past to apply coatings to paper, either directly, or by first applying to a roll, as in a size press. Once coating has been applied to the substrate, it is necessary to meter the coating to a desired thickness and uniform level. Uneven coating thickness will produce blemishes and quality variances in the finished paper, and is highly undesirable.
One approach to metering the coating is to position a flexible blade against the backing roll downstream of the coating pond. Blades, however, directly engage the coating, and in some applications, blade defects may be telegraphed to the coating, causing corresponding defects in the paper. Often a fiber will be caught on the blade and cause a nonuniform coating or stripe.
Rod metering devices have a rod which extends in the cross-machine direction the length of the substrate. One conventional rod holder utilizes a press fit between a rod holder housing and the rod. This press fit of the rod to the holder ensures an effective seal between the housing and the rod to prevent leakage of the cleaning and lubricating fluid which must be introduced to the rod. However, a tight fit places heavy demands on the motor which rotates the rod, and the friction between the rod and the housing causes rapid wear on the rod metering device, and hence necessitates frequent replacement. Down time of a coating apparatus to serve the rod metering applicator is costly, because production of paper must stop, and should be minimized.
What is needed is a rod holder which is long lasting, rapidly serviceable, and which at the same time holds the rod firmly, maintains an effective seal between the rod and the holder, and allows easy rotation of the rod.